With many sweltering days ahead, people are urged to stay safe, cool and hydrated.

The first heat wave of the summer hit California on Thursday, producing some triple-digit temperatures that forecasters say are only a preview for a blistering weekend ahead.

The National Weather Service issued heat warnings for large swaths of the state, saying many inland valleys and desert areas could see temperatures well above 100 degrees for the next several days. The mercury could top 120 degrees in the Coachella Valley and 129 in Death Valley, still short of the 134-degree record set there in 1913.

The heat is a particular concern to firefighters because it comes in a year of record dry conditions that have already sparked several major brush fires across Southern California. On top of that, fireworks go on sale in some areas beginning Friday, adding another fire danger.

Fireworks are to be sold in 295 designated communities in the state through the Fourth of July.

Since January, the California Department of Forestry and Fire Protection has responded to about 2,900 fires, spokesman Daniel Berlant said. In an average year, he said, it would have responded to fewer than 1,800 by this time.

For The Record Los Angeles Times Saturday, June 29, 2013 Home Edition Main News Part A Page 4 News Desk 1 inches; 52 words Type of Material: Correction Heat wave: An article in the June 28 LATExtra section about a heat wave in Southern California incorrectly quoted climatologist Bill Patzert as saying that California receives 15.1 inches of rain on average and has gotten only 5.85 inches so far this year. Those statistics were for Los Angeles, not the state.

This increase in fire starts results from the prevalence of dry brush, Berlant said. He added that current weather conditions are more typical of late August or early September.

"We're in a long-term drought," said Bill Patzert, a climatologist with the NASA Jet Propulsion Laboratory in La Canada Flintridge. "The situation is extremely crispy and dry. That equals incendiary."

During a typical rain year, which runs from July 1 through June 30, California receives 15.1 inches of rain on average, Patzert said. During this rain year, which ends Sunday, the state has seen only 5.85 inches so far, making it the sixth-driest of the last 135 years, according to Patzert.

The high temperatures combined with low humidity and dry brush increase the danger of wildfires, said Bonnie Bartling, a weather specialist at the National Weather Service's Los Angeles station. But because high winds are not forecast for most areas, the service has issued a fire warning only in southern Santa Barbara County.

Somewhat lower temperatures are expected starting Monday, but most residents still will find it plenty hot. " 'Cooler' is a relative term," Bartling said.

Because of the rising heat levels and low humidity, the U.S. Forest Service has extended the hours of its staffing in the Angeles National Forest, effective at least through Monday.

Personnel will be on high alert July 4, although fireworks are never permitted in the park, spokesman Nathan Judy said. Some campgrounds may close early that day.

"If we have fires, the chance of their growing larger is that much greater," Judy said.

The Los Angeles County Fire Department is increasing its staffing Tuesday in anticipation of the holiday, spokesman Tony Akins said. Peak staffing of about 90 firefighters is scheduled for the Fourth of July.

Both the county and city fire departments support an anti-fireworks campaign, urging people to attend public fireworks shows rather than hold their own.

Officials have issued lists of recommendations: Don't leave pets or people in closed cars, drink plenty of water, and avoid consuming too much alcohol, which is dehydrating. People also are urged to stay indoors during the hottest part of the day, to avoid strenuous physical activity outdoors and to call 911 at the first sign of heatstroke, which can be fatal.

The heat wave could be especially dangerous for the elderly, small children and people with chronic ailments, authorities said.

Several agencies opened cooling centers -- air-conditioned public facilities that can be used to escape the heat. Information about the centers can be found by dialing 211, the county's information line.

Parts of Northern California -- especially inland areas -- will also be slammed by the heat. Temperatures in Sacramento could reach 108 degrees, but they are not expected to surpass 75 in San Francisco.